The invention pertains to a method of polishing a metal layer of a substrate using a chemical-mechanical polishing (xe2x80x9cCMPxe2x80x9d) system.
Compositions and methods for planarizing or polishing the surface of a substrate are well known in the art. Polishing compositions (i.e., polishing slurries) typically contain an abrasive material in an aqueous solution and are applied to a surface by contacting the surface with a polishing pad saturated with the slurry composition. Typical abrasive materials include silicon dioxide, cerium oxide, aluminum oxide, zirconium oxide, and tin oxide. U.S. Pat. No. 5,527,423, for example, describes a method for chemically-mechanically polishing a metal layer by contacting the surface with a polishing slurry comprising high purity fine metal oxide particles in an aqueous medium. Alternatively, the abrasive material may be incorporated in to the polishing pad. U.S. Pat. No. 5,489,233 discloses the use of polishing pads having a surface texture or pattern, and U.S. Pat. No. 5,958,794 discloses a fixed abrasive polishing pad.
Conventional polishing systems and polishing methods typically are not entirely satisfactory at planarizing semiconductor wafers. In particular, polishing slurries and polishing pads can have less than desirable polishing rates, and their use in chemically-mechanically polishing semiconductor surfaces can result in poor surface quality. Because the performance of a semiconductor wafer is directly associated with the planarity of its surface, it is crucial to use a polishing method that has a high polishing efficiency, uniformity, and removal rate and leaves a high quality polish with minimal surface defects.
The difficulty in creating an effective polishing system for semiconductor wafers stems from the complexity of the semiconductor wafer. Semiconductor wafers are typically composed of a substrate, on which a plurality of transistors has been formed. Integrated circuits are chemically and physically connected into a substrate by patterning regions in the substrate and layers on the substrate. To produce an operable semiconductor wafer and to maximize the yield, performance, and reliability of the wafer, it is desirable to polish select surfaces of the wafer without adversely affecting underlying structures or topography. In fact, various problems in semiconductor fabrication can occur if the process steps are not performed on wafer surfaces that are adequately planarized.
There have been many attempts to improve the polishing efficiency and uniformity of conventional polishing agents, while minimizing defectivity of the polished surface and damage to underlying structures or topography. For example, U.S. Pat. No. 5,264,010 describes a polishing composition comprising cerium oxide, fumed silica, and precipitated silica, which purportedly yields an improved removal rate and polishing efficiency. U.S. Pat. No. 5,114,437 describes a polishing composition comprising a carrier, alumina, and a polishing accelerator selected from chromium(III) nitrate, lanthanum nitrate, ammonium cerium nitrate, and neodymium nitrate. U.S. Pat. No. 6,110,396 describes a polishing composition comprising abrasive particles and dual-valent rare earth ions in their higher valent form at acidic pH. U.S. Pat. No. 6,143,192 describes a method of removing ruthenium or ruthenium dioxide using a solution comprising water, ammonium cerium nitrate, and acetic acid. Japanese Laid Open Patent Application 2000167764 describes an improved ruthenium removal rate using slurry comprising ammonium cerium nitrate.
A need remains, however, for polishing systems and polishing methods that will exhibit desirable planarization efficiency, uniformity, and removal rate during the polishing and planarization of substrates, while minimizing defectivity, such as surface imperfections and damage to underlying structures and topography during polishing and planarization. Improved polishing systems are particularly needed for the polishing of noble metal-containing substrates, since noble metals are know to be chemically stable and mechanically hard materials.
The present invention seeks to provide such a chemical-mechanical polishing system and method. These and other advantages of the present invention will be apparent from the description of the invention provided herein.
The invention provides a method of polishing a substrate comprising (i) contacting a substrate comprising at least one metal layer with a chemical-mechanical polishing system comprising an abrasive and/or polishing pad, a rare earth salt, an oxidizer that is a stronger oxidant than the rare earth salt, and a liquid carrier and (ii) abrading at least a portion of the metal layer of the substrate to polish the metal layer.
The invention is directed to a method of polishing a metal layer of a substrate using a chemical-mechanical polishing system comprising an abrasive and/or polishing pad, a rare earth salt, an oxidizer that is a stronger oxidant than the rare earth salt, and a carrier. The abrasive (when present and suspended in the liquid carrier), rare earth salt, oxidizer, and liquid carrier, as well as any other components suspended in the liquid carrier, form the polishing composition of the CMP system.
The chemical-mechanical polishing system comprises an abrasive, a polishing pad, or both. Preferably, the CMP system comprises both an abrasive and a polishing pad. The abrasive can be any suitable abrasive. The abrasive can be fixed on the polishing pad and/or can be in particulate form and suspended in the liquid carrier. The polishing pad can be any suitable polishing pad.
The abrasive is any suitable abrasive known in the art. For example, the abrasive particles are natural or synthetic and include diamond (e.g., polycrystalline diamond), garnet, glass, carborundum, metal oxide (e.g., silica, fused alumina, ceramic alumina, chromia, and iron oxide), and the like. The abrasive particles may be coated particle abrasives. The abrasive preferably is a metal oxide abrasive and more preferably is selected from the group consisting of alumina, silica, titania, ceria, zirconia, germania, magnesia, co-formed products thereof, and combinations thereof. Most preferably, the abrasive is alumina.
When the abrasive is present in the CMP system and is suspended in the liquid carrier (i.e., when the abrasive is a component of the polishing composition), any suitable amount of abrasive can be present in the polishing composition. Typically, about 0.1 wt. % or more (e.g., about 0.5 wt. % or more) abrasive will be present in the polishing composition. More typically, about 1 wt. % or more abrasive will be present in the polishing composition. The amount of abrasive in the polishing composition typically will not exceed about 30 wt. %, more typically will not exceed about 20 wt. % (e.g., will not exceed about 10 wt. %).
The rare earth salt can be any salt comprising a rare earth cation (i.e., a lanthanide or actinide metal ion) and a suitable counter-anion. Preferably, the rare earth salt comprises a rare earth (RE) cation, in an oxidation state of RE2+, RE3+, or RE4+, and a counter-anion. The rare earth cation desirably is selected form the group consisting of lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium ions, as well as combinations thereof. The counter-anion desirably is selected from the group consisting of sulfate, nitrate, carbonate, hydroxide, fluoride, chloride, bromide, iodide, acetate, perchlorate, oxalate, acetylacetonate, and trifluoromethanesulfonate.
Preferably, the rare earth salt is a cerium or praseodymium salt. More preferably, the rare earth salt is a cerium (II), cerium (III), or cerium (IV) salt. Most preferably, the rare earth salt is cerium (III) acetate.
Any suitable amount of the rare earth salt can be present in the polishing composition. Typically, about 1xc3x9710xe2x88x927 M or more rare earth salt will be present in the polishing composition. More typically, about 1xc3x9710xe2x88x926 M or more (e.g., about 1xc3x9710xe2x88x925 M or more) rare earth salt will be present in the polishing composition. The amount of rare earth salt in the polishing composition typically will not exceed about 0.1 M, more typically will not exceed about 0.05 M (e.g., will not exceed about 0.02 M).
The oxidizer is any suitable oxidizer capable of oxidizing the rare earth salt. Per-type oxidizers are particularly suitable, including inorganic or organic per-compounds. A per-compound (as defined by Hawley""s Condensed Chemical Dictionary) is a compound containing at least one peroxy group (xe2x80x94Oxe2x80x94Oxe2x80x94) or a compound containing an element in its highest oxidation state. Examples of compounds containing at least one peroxy group include but are not limited to hydrogen peroxide and its adducts such as urea hydrogen peroxide and percarbonates, organic peroxides such as benzoyl peroxide, peracetic acid, and di-tert-butyl peroxide, monopersulfates (SO52xe2x88x92), dipersulfates (S2O82xe2x88x92), and sodium peroxide. Examples of compounds containing an element in its highest oxidation state include but are not limited to periodic acid, periodate salts, perbromic acid, perbromate salts, perchloric acid, perchlorate salts, perboric acid, perborate salts, and permanganates. The per-type oxidizer preferably is hydrogen peroxide. Other suitable oxidizers include bromates, chlorates, chromates, and iodates, with any appropriate countercation, as well as iron salts (e.g., nitrates, sulfates, EDTA, and citrates), potassium ferricyanide, potassium dichromate, iodic acid, and the like.
Any suitable amount of the oxidizer can be present in the polishing composition. Typically, about 0.01 wt. % or more (e.g., about 0.1 wt. % or more) oxidizer will be present in the polishing composition. More typically, about 0.2 wt. % or more (e.g., about 0.5 wt. % or more) oxidizer will be present in the polishing composition. The amount of oxidizer in the polishing composition typically will not exceed about 10 wt. %, more typically will not exceed about 5 wt. % (e.g., will not exceed about 2 wt. %).
The presence of an oxidizer in conjunction with the rare earth salt in the context of the inventive chemical-mechanical polishing system appears to exhibit a synergistic effect, which improves the polishing rate of the substrate. While not wishing to be bound to this theory, it is believed that the rare earth ion acts as a catalyst in the reaction of the oxidizer with the metal surface. For example, the surface of the substrate can be oxidized by a Ce(IV) compound which in turn is reduced to Ce(III). The oxidizer, for example, hydrogen peroxide, then re-oxidizes the Ce(III) ion to Ce(IV) resulting in a higher percentage of Ce(IV) overall during the polishing process. The proposed mechanism is supported by the fact that, for example, Ce(III) acetate, although itself not an oxidizer, is equally effective in the removal of metal from a substrate as a cerium(IV) salt when used in combination with an oxidizer. The choice of rare earth salt and oxidizer should be such that the oxidizer is a stronger oxidant than the rare earth salt, i.e., an oxidizer that is capable of spontaneously oxidizing the rare earth salt under the conditions of the chemical-mechanical polishing system. Such oxidizers will have an oxidation potential greater in magnitude that the oxidation potential of the rare earth salt.
A liquid carrier is used to facilitate the application of the abrasive (when present), rare earth salt, and oxidizer to the surface of a suitable substrate to be polished or planarized. The liquid carrier can be any suitable liquid carrier. Preferably, the liquid carrier comprises, consists essentially of, or consists of water, more preferably deionized water.
The chemical-mechanical polishing system optionally further comprises one or more solutes, for example, acetic acid or other liquid or solid solutes. Such solutes desirably act as agents for buffering or enhancing dissolution of the rare earth salt in the liquid carrier. Such solutes can be present in the polishing composition in any suitable amount (e.g., 0.1-5 wt. %).
The chemical-mechanical polishing system also optionally further comprises an amine-containing polymer or copolymer, for example, polyethylenimine, polyetheramine, polydiallyldimethylammonium chloride (polydadmac), and mixtures thereof. Such polymers and copolymers can be present in the polishing composition in any suitable amount (e.g., 0.1-5 wt. %).
The chemical-mechanical polishing system preferably comprises about 1-10 wt. % carrier-suspended abrasive particles, about 1xc3x9710xe2x88x927 Mxe2x88x920.05 M rare earth salt, about 0.1-5 wt. % oxidizer, and water. In a more preferred embodiment, the CMP system comprises alumina particles, cerium acetate, hydrogen peroxide, and water.
The chemical-mechanical polishing system desirably is used in a method of polishing a substrate comprising at least one metal layer, whereby the substrate is contacted with the chemical-mechanical polishing system and at least a portion of the metal layer of the substrate is abraded such that the metal layer becomes polished. The substrate can be any suitable substrate (e.g., an integrated circuit, rigid memory disk, or magnetic head) and can contain any suitable metal or metal alloy (e.g., metal conductive layer). The CMP system is particularly well suited for polishing noble metal-containing substrates, especially those used in the electronics industry. The substrate preferably comprises a noble metal selected from the group consisting of rhenium, ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, and gold. In a more preferred embodiment, the noble metal is ruthenium.